With reference to FIG. 1, a ducted fan gas turbine engine generally indicated at 10 has a principal and rotational axis X-X. The engine comprises, in axial flow series, an air intake 11, a propulsive fan 12, an intermediate pressure compressor 13, a high-pressure compressor 14, combustion equipment 15, a high-pressure turbine 16, and intermediate pressure turbine 17, a low-pressure turbine 18 and a core engine exhaust nozzle 19. A nacelle 21 generally surrounds the engine 10 and defines the intake 11, a bypass duct 22 and a bypass exhaust nozzle 23.
The gas turbine engine 10 works in a conventional manner so that air entering the intake 11 is accelerated by the fan 12 to produce two air flows: a first air flow A into the intermediate pressure compressor 13 and a second air flow B which passes through the bypass duct 22 to provide propulsive thrust. The intermediate pressure compressor 13 compresses the air flow A directed into it before delivering that air to the high pressure compressor 14 where further compression takes place.
The compressed air exhausted from the high-pressure compressor 14 is directed into the combustion equipment 15 where it is mixed with fuel and the mixture combusted. The resultant hot combustion products then expand through, and thereby drive the high, intermediate and low-pressure turbines 16, 17, 18 before being exhausted through the nozzle 19 to provide additional propulsive thrust. The high, intermediate and low-pressure turbines respectively drive the high and intermediate pressure compressors 14, 13 and the fan 12 by suitable interconnecting shafts.
Leaf seals are formed from sections of leaf material appropriately presented in order to create a seal surface from juxtaposed leaf edges of respective leaves in an assembly. Typically the leaves are arranged circumferentially about a rotating shaft in order to present the leaf edges and therefore the seal surface towards that shaft in order to provide a seal barrier. Typically, spacer members are provided between each leaf in order to correctly arrange the seal elements for presentation of the leaf edges and therefore the seal surface.
In a gas turbine engine, leaf seals may be used to form a seal between a static component and a rotating component, between two relatively rotating components, or even between two static components in order to maintain a relatively high pressure on one side of the seal and relatively low pressure on the other. FIG. 2, which shows schematically, for example, a cut-away perspective view of a portion of a leaf seal assembly 31 comprises leaves 32 extending from spacer elements 33 secured in a housing comprising a backing ring 34 with coverplates 35. The leaves 32 present leaf edges 36 towards a sealing surface 37 of a rotating component generally rotating in the direction depicted by arrowhead 38. The leaves 32, and in particular the leaf edges 36 of the leaves 32, make wiping contact with the surface 37 in order to create a seal across the assembly 31. Each leaf 32 is generally compliant in order to adjust with rotation and radial and axial movement of the surface 37 to ensure that a good sealing effect is created. The spacers 33 are generally required in order to ensure that flexibility is available to appropriately present the leaves 32 towards the surface 37 which, as illustrated, is generally with an inclined angle between them.
The spacers 33 may be separate components interposed between the root portions of the leaves 32, or they may be formed by folding over extra material at the root portions of the leaves 32. The leaves 32 may be bent e.g. where they meet the spacers 33, to change the angle that the leaves make with the radial direction. By adjusting the spacers 33 between the leaves, and/or by changing the bend angle, the inter-leaf spacing and/or the leaf lay angle can be changed.
Effectively the pack geometry is governed by six parameters (some of which are illustrated in FIG. 3):                Leaf thickness T to inter leaf gap G ratio.        The radius R at which the leaves meet the spacers (often called the “weld radius” as the leaves are typically bonded together by welding, bonding or brazing at this position).        Number of leaves.        Leaf lay angle α (i.e. the angle between the leaf at the weld radius and a radial line).        Leaf length L (i.e. the length of the leaf from the weld radius to the leaf edges making wiping contact)        Leaf axial length.        
Other parameters, such as leaf thickness, and radius at the leaf tip, can be calculated once the above values are known.
During seal manufacture it is not possible to achieve the six parameters defining the leaf pack geometry exactly. This can be due to variations in the raw material (e.g. thickness of leaf material) and due to variations that arise during formation processes during the manufacture of the seal (e.g. securing the leaves in the housing, or bending the leaves). Consequently a batch of leaf seals will have a range of seal bore radii (i.e. radii at the leaf edges 36). In a large enough batch, the bore radii are generally normally distributed about the nominal radius.
Due to the range of bore radii, when fitting the seals on a standard size rotor, a number of the seals will have interference (i.e. a seal bore radius smaller than the radius of the sealing surface of the rotor) and will suffer from excessive rub and heat generation which may lead to damage of the rotor and/or the seal. Others seals, however, will operate at a clearance (i.e. a seal bore radius larger than the radius of the sealing surface of the rotor) and will have a poor leakage characteristic as air will leak through the gap. Both these cases are undesirable and may result in seals having to be rejected.